Concept II is an industrial grade, wind resistance rower. You get on the sliding seat, strap your feet to the footrests, and start pulling on the handle that replaces oars. The Model D we have at the gym has the capability to race with other rowers: it is equipped with a PM4 monitor which will show you your progress as you row competing with another machine (the other machine has to have a PM4 monitor as well). Racing is probably one of the best features. although you have to have more than one rower and another user to compete with. You can also race against your own saved previous workouts.
The monitor also shows all the traditional data like speed, time, distance, burned calories, heart rate, etc. There are 10 resistance levels on this machine, and finding the right one to match your strength is the key to not over-straining yourself; sore muscles all over on the next day are no fun.
I used to find this machine nice and smooth... until I had tried a water resistance rower. Water resistance gives you a more natural, gliding feel. Now whenever I'm on Concept II, I can't help seeing the difference, noticing higher level of noise and mechanical jolts. So while Concept II is definitely a good machine, it is no longer my favorite.